Hair styling iron

ABSTRACT

A hair styling appliance has a temperature regulation controller to mitigate heat damage to the hair. The appliance includes a pair of heated jaws that can be closed to grip the hair. A power control circuit responds to an activation sensor that senses when the jaws are closed, and a temperature sensor that senses the temperature of a heating element on the jaws. The power control circuit, in response to an activation sensor output, starts monitoring the rate of change of the heating element temperature, and, in response to a rate of decrease above a predetermined level, operates the heating element at a power level lower than a steady state power level.

TECHNICAL FIELD

The present invention relates to electric irons for thermal shaping and styling of hair, and particularly to the temperature regulation of hair styling irons.

BACKGROUND OF THE INVENTION

Hair irons used for curling or straightening of hair employ heating elements which are typically plate or barrel-shaped with outer surfaces which contact the hair. The heating elements have relatively low thermal mass, making the irons convenient to use, because they may reach operating temperature within fractions of a minute. The elements are thermostatically controlled, whereby hair held against the heating element applies a thermal load that tends to reduce the temperature of the heating element, in response to which a thermostat increases power to attempt to restore the setpoint temperature. A drawback with hair styling irons employing such conventional temperature regulation techniques is a power consumption penalty which, particularly in the case of battery powered hair irons, has significant performance impacts.

As the use of low operating temperatures increases styling time, or else produces poor styling results, the temperature should be set at the highest level that avoids heat damage to the hair. However, this is not a simple matter as the amount of heat transferred when excessive processing times are used can also result in heat damage. A number of other variables beyond the time-temperature profile also affect the potential for heat damage things such as the intrinsic properties of an individual's hair, treatments agents applied to the hair (such as water or other softening agents), as well as the manner in which the hair iron is used (the size of a tress which is treated, the tension applied to the hair etc). As a result, it is difficult for the user to select the correct temperature for any specific operation that mitigates the possibility of heat damage.

The patent publication US20120005550 describes a method of temperature regulation in a hair straightening iron, in which two temperature sensors are provided for sensing the temperature of the hair. The two sensors are mounted to the heating element on opposing sides such that, as the appliance is drawn along the hair, one sensor measures the temperature of the hair about to be treated, while the other measures the temperature of the hair that has just been treated. However, in practice, it is very difficult to obtain a measureable difference between the outputs of the two sensors as, being mounted to the same element, the output of both sensors is overwhelmed by the heat output from the element.

The patent publication US20110253164 also describes a method of temperature regulation in a hair straightening iron, having two jaws for gripping the hair. A sensor detects closure of the jaws and increases the setpoint temperature for the heating elements. However, by boosting the power to the heating elements in this way, this method does not address the questions of power saving or the avoidance of heat damage.

It is an object of the present invention to overcome or substantially ameliorate the above disadvantages without adversely affecting styling performance or, more generally, to provide an improved hair styling iron.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention there is provided a hair styling appliance comprising:

-   -   at least one heating element having a heating surface for         contact with the hair;     -   a pair of jaws, the heating element being disposed on one of the         jaws, the jaws being connected for movement between an open         position for inserting or removing hair between the jaws, and a         closed position in which the other of the jaws presses the hair         against the heating surface;     -   a temperature sensor that senses the temperature of the heating         element proximate the heating surface;     -   an activation sensor that senses when the jaws are in the closed         position and generates an first activation output;     -   a power control circuit that responds to the activation sensor         and the temperature sensor, and is operative to operate the         heating element at a steady state power level to maintain the         temperature in a steady state;     -   the power control circuit including means for determining a time         rate of decrease of the temperature of the heating element, and         means for monitoring the temperature and the time rate of         decrease of the temperature to detect an abnormal thermal load         on the heating element;     -   the power control circuit being operative in response to the         first activation output to start detection of an abnormal         thermal load, and in response to an abnormal thermal load to         operate the heating element at a power level lower than the         steady state power level.

The steady state power level is a power level that results in a steady state element temperature, or an equivalent thereof. For instance, either a constant power level or, if a steady state element temperature is attained by a power level that oscillates instantaneously, then the time-average of this oscillating power level may be considered a steady state power level. The power level lower than the steady state power level may be a zero power level.

Optionally, the appliance may further include an input device for enabling the user to select one of a plurality of power settings for the heating element, the control circuit being operatively connected to the input device and operative to operate the heating element at a steady state power level corresponding to the user selected power setting and, in response to detection of an abnormal thermal load, to operate the heating element at a power level lower than the steady state power level associated with the user selected power setting.

Preferably the means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load comprises means for comparing the measured time rate of decrease of the temperature to a predetermined rate of decrease representing an abnormal thermal load, the monitoring means detecting an abnormal thermal load when the measured time rate of decrease is lower than the predetermined rate of decrease. Optionally, as hair is not damaged by temperatures below a threshold temperature, the means for monitoring the temperature and the time rate of decrease of the temperature comprises means for comparing the sensed temperature to a predetermined threshold temperature, the monitoring means only detecting an abnormal thermal load when the sensed temperature exceeds the predetermined threshold temperature.

Preferably the means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load comprises means for comparing the sensed temperature to a first calculated temperature, a second calculated temperature lower than the first calculated temperature, and a third calculated temperature lower than the second calculated temperature and means for comparing the measured rate of decrease of temperature to a first predetermined rate of decrease, a second predetermined rate of decrease less than the first rate of decrease and a third predetermined rate of decrease less than the second rate of decrease, the monitoring means detecting an abnormal thermal load when the sensed temperature is greater than the first calculated temperature and the measured rate of decrease is less than the first reference rate of decrease or when the sensed temperature is less than the second calculated temperature and the measured rate of decrease is less than the second reference rate of decrease or when the sensed temperature is less than the third calculated temperature and the measured rate of decrease is less than the third reference rate.

The power control circuit may be operative to reduce the power level applied to the heating element by the same amount when the sensed temperature is greater than the first reference temperature and the measured rate of decrease is less than the first reference rate of decrease or when the sensed temperature exceeds the second reference temperature and the measured rate of decrease is less than the second reference rate of decrease or when the sensed temperature exceeds the third reference temperature and the measured rate of decrease is less than the third reference rate. Alternatively, the power control circuit may be operative to reduce the power level applied to the heating element by a first amount when the sensed temperature exceeds the first reference and the measured rate of decrease is less than the first reference rate of decrease or when the sensed temperature exceeds the second reference and the measured rate of decrease is less than the second reference rate, or when the sensed temperature exceeds the third reference rate and the measured rate of decrease is less than the third reference rate of decrease, and to reduce the power level by a second amount greater than the first amount when the sensed temperature exceeds the second reference temperature and the measured rate is less than the first reference rate or when the measured temperature exceeds the third reference temperature and the measured rate is less than the second reference rate and to reduce the power level applied to the heating element by a third amount greater than the second amount when the sensed temperature exceeds the third reference temperature and the measured rate is less than the first reference rate.

Preferably the means for measuring the temperature rate of decrease comprises timing means for establishing the time interval between successive rate of decrease measurements; the timing means establishing first, second and third time intervals measured from the time of the first activation output, the measured rate of decrease occurring at the end of the first, second and third time intervals being compared with the first, second and third predetermined rates of decrease respectively.

Preferably the power control circuit stores a reference temperature corresponding to the steady state power output, and the first, second and third calculated temperatures are calculated by subtracting predefined first, second and third reference values from the reference temperature to define the first, second and third predetermined rates of decrease.

Preferably the control circuit operates the heating element in a thermostatic mode at first power levels that vary as a function of a temperature offset to maintain a setpoint temperature, the temperature offset comprising the difference between the instantaneous element temperature and the setpoint temperature and wherein the control circuit comprises means for comparing the rate of decrease to a reference rate of decrease, and when the measured rate of decrease is greater than the reference rate of decrease, the control circuit operates the heating element in a boost mode at power levels higher that the first power levels at all temperature offsets until there is detected a measured rate of decrease of heating element temperature less than the reference rate or a heating element temperature at a setpoint temperature corresponding to the steady state power output.

Preferably the activation sensor generates a second activation output when the jaws are in the open position and the power control circuit is further operative in the presence of the second activation output and following the operation of the heating element at the power level lower than the steady state power level to operate the heating element in the boost mode until the applied power level again equals the steady state power level or until detection of an abnormal thermal load recurs.

In another aspect the invention provides a method of controlling power in a hair styling appliance of the type having a heating element and a temperature sensor that senses the temperature of the heating element proximate a heating element surface, a pair of jaws, the heating element being disposed on one of the jaws, the jaws being connected for movement between an open position for inserting or removing hair between the jaws, and a closed position in which the other of the jaws presses the hair against the heating element, and an activation sensor that senses when the jaws are in the closed position and generates an first activation output; the power control method comprising the steps of:

-   -   operating the heating element at a steady state power level;     -   when the first activation output is generated, periodically         measuring the temperature and computing a time rate of decrease         of the temperature and comparing the computed rate to a         predetermined rate, and     -   when the measured rate of decrease of the temperature is less         than the predetermined rate, reducing the power level applied to         the heating element to protect against hair damage by         overheating.

This invention provides a hair styling iron which is effective and efficient in operational use, and which has an overall simple design which minimizes manufacturing costs. Responding according to the measured rate of change of temperature, power savings are achieved by lowering power to the elements when abnormally low thermal loads are detected, and by avoiding excessive temperatures in this same way the possibility of heat damage to the hair is mitigated.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic sectional illustration of a hair iron according to an embodiment of the invention;

FIG. 2 is a functional block diagram of the electrical circuitry for the hair iron of FIG. 1, and

FIGS. 3, 4 a, 4 b and 5 are graphic representations of the temperature and electrical current vs. time characteristics for the heating elements of the hair iron of FIG. 1, and temperature vs. time for hair being heated by the same.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 illustrates a first embodiment of a hand-held electric hair iron having first and second elongate jaws 10, 11 which may be joined at their proximal ends. Both jaws 10, 11 may be of hollow moulded polymeric construction, and include respective elongate heating elements 13, 14 on their distal ends. The heating elements 13, 14 are assemblies that include outer members 16, 17 made of metal, such as chromium steel. Respective heating surfaces 16 a, 17 a of the outer members 16, 17 are complementary, generally planar inner surfaces that may be polished. The surfaces 16 a, 17 a oppose one another and are configured to be held in contact with the hair. The outer members 16, 17 are heated by respective internal resistive elements 15 a, 15 b arranged in a flat configuration and disposed directly inside the outer members 16, 17. Power may be provided to the heating elements 13, 14 from a power control circuit 20 via a power supply 18 such as an electrical cord connected to mains power. The jaws 10, 11 may be connected by a hinge 12, or any like mechanism, whereby the jaws 10, 11 are connected for movement between an open position (see FIG. 1) for inserting or removing hair between the jaws, and a closed position (not shown) in which the other of the jaws 10, 11 grip the hair, pressing it between the heating surfaces 16, 17.

Also connected to the power control circuit 20 is a temperature sensor 21 and an activation sensor 22. The temperature sensor 21 senses the temperature of the heating element assembly 14 proximate the heating surface 17 a and may be a may be a thermistor or thermocouple, for example. The activation sensor 22 senses when the jaws 10, 11 are in the closed position, as by mechanical, optical or magnetic means and generates an first activation output that is transmitted to the power control circuit 20, and correspondingly provides a second activation output when the jaws are closed.

Not shown in FIG. 1, but shown in simplified schematic form in FIG. 2 is an input device 25, such as a push-button switch for enabling the user to select one of a plurality of discrete power settings for the heating elements 13, 14 and a display 26 to show the power setting selected by the user. The power control circuit 20 drives power through the heating elements 13, 14 based on feedback from the temperature sensor 21, as by switching on and off the current supplied to the heating elements 13, 14. Each of the power settings has uniquely associated with it a particular steady state power level, allowing a particular temperature setpoint or setpoint range to be maintained when not in use.

It is important to limit the operating temperature of the heating elements 13, 14 to avoid heat damage to the hair. The main heat transfer mechanism in the hair iron is by conduction from the heating elements 13, 14 to the hair. A typical 4 grammes of dry hair, firmly gripped, makes good contact with the heating surfaces 16 a, 17 a and provides good heat transfer, as the appliance is drawn along the hair. When so used, the temperature of the heating elements 13, 14 will usually remain within acceptable limits without need for any corrective action.

However, excessive temperatures of the heating elements 13, 14 can occur as a result of abnormally low loads which do not provide adequate heat transfer. Commonly occurring abnormally low thermal loads likely to cause excessive heating can be caused, for instance, by treating a reduced amount of hair, or increasing the time which heat is applied to any section of the hair.

It will be recalled that an object of the present invention is to protect against hair damage from overheating without adversely affecting styling performance. To that end, in accordance with one aspect of the invention, the power control circuit uses heating element temperature information to anticipate the occurrence of an over-temperature condition by detecting the presence of an abnormally low load and adjusting the power level applied to the heating element before the temperature exceeds acceptable limits. By anticipating the condition before it actually exists, power level adjustments can be made gradually, having less affect on styling performance.

It has been determined that when common abnormally low thermal loads of the type outlined above are applied to the heating elements 13, 14 with the heating surfaces 16 a, 17 a are heated to operating temperatures between 180 and 250 degrees C., the temperature of the heating surfaces 16 a, 17 a decreases at a rate lower than the rate characteristic of most normal loads. Advantageous use is made of this phenomenon in the control system of the present invention by measuring the rate of decrease of temperature of heating elements 13, 14 and comparing this rate to a predetermined rate characteristic of an abnormally low thermal load. When the measured rate is lower than the reference rate, signifying the presence of an abnormally low thermal load on the hair iron, power to the heating element is reduced to a lower level. By this arrangement the temperature of the heating surfaces 16 a, 17 a is limited so as to avoid overheating and resulting hair damage.

Since it is desirable to maintain a steady state temperature before use, and this is maintained within a temperature band. At any instant, the rate of decrease of the temperature of the heating elements 13, 14 may be lower than the predetermined rate characteristic of an abnormally low thermal load, so this case needs to be distinguished from the case where this low rate of decrease occurs during operation. This is achieved, using an activation sensor that senses when the jaws 10, 11 are in the closed position and generates the first activation output. Once the first activation output has been received, the power control circuit 20 may begin monitoring the rate of decrease of temperature.

A series of tests were conducted using the hair styling appliance of the illustrative embodiment with different heating loads to generate temperature vs. time curves characterising the various different loads. The tests were conducted over a range of starting steady stare temperatures from 120 to 250° C., using loads provided by hair emulators in the form of conductive metal (Aluminium) strips. Depending upon the gauge of the strip used, the strips copy or emulate the temperature versus time characteristics of bulk lengths of dry hair. A 4 gramme and 2 gramme hair emulator were used, to emulate 4 gramme and 2 gramme lengths of hair of 200 mm length. A typical operating cycle was considered, whereby at a selected steady state power level a steady state temperature was first obtained, before closing the jaws on the emulator and drawing the appliance from one end of the emulator to the other in five seconds, and then releasing the hair. An instantaneous surface temperature on the emulator of 155° C., or more, was the threshold beyond which some overheating damage to the hair was found.

From the resulting curves and analysis a characteristic reference rate of decrease was established for abnormally low thermal loads. Satisfactory styling results have been achieved in the illustrative embodiment using the time rates of change between −10° C./sec and −8° C./sec. Specifically, measured from the first activation output, a drop of 10° C. in the first elapsed second, or a drop of 19° C. after the second second, or a drop of 27° C. by the third elapsed second, were found satisfactory, while rates of decrease less than these figures showed the potential to result in overheating. The reference temperatures are selected with a view to allowing for corrections early enough to prevent overheating with the least possible reduction in power level but late enough to avoid unnecessary adjustments. The values for the reference rates and reference temperatures have been empirically determined to provide good results with the appliance of the illustrative embodiment, but it is to be understood that they are intended to illustrate but not limit the invention.

FIGS. 3, 4 a, 4 b and 5 illustrate the variations with time in the temperature of the heating elements 13, 14 (uppermost), the temperature of the different hairemulators (intermediate), and the current supplied to the heating elements 13, 14 (lowermost). The current supplied to the heating elements 13, 14 is directly proportional to the power, since the voltage is constant, so controlling the current controls the power level. As shown, the steady state temperature is maintained when the iron is not in use by a steady state power or current level. While the instantaneous current supplied to the elements may actually vary to maintain the elements at a steady state temperature, the time averaged current or power level is constant for a given steady state temperature. Prior to operation, the heating elements 13, 14 are held at the 200° C. setpoint by the power control circuit 20 at a steady state power output at which about 1 ampere flows through the elements 13, 14 and the hair emulator is at room temperature. The heating elements 13, 14 attain this temperature with the power control circuit 20 operating in a thermostatic control mode. In the thermostatic control mode the control circuit operates at thermostatic power levels that vary as a function of the temperature offset or difference between the instantaneous element temperature and the setpoint temperature i.e. the greater the temperature offset higher the power level. The thermostatic power level for a given temperature offset may be determined, for instance, by a lookup table, or by a calculation performed by the power control circuit 20.

FIG. 3 illustrates operation under a normal heat load, comprising a 4 gramme hair emulator. At the 10 second mark the jaws are clamped onto the emulator, and the power control circuit 20 receives the first activation output, starting a timer and recording the starting temperature. After 1 second has elapsed on the timer, the power control circuit 20 reads the instantaneous heating element temperature and evaluates the first rate of decrease by subtracting the current temperature from the starting temperature and dividing the result by the elapsed time. This first rate of decrease is then compared against the predetermined 10° C./sec rate of decrease, which is stored in memory in the circuit. As the measured time rate of decrease is equal or greater than the predetermined rate of decrease, the power control circuit 20 responds to increase the current to the heating elements 13, 14 according to the thermostatic power level determined by the power control circuit 20 in proportion to the difference between the instantaneous temperature and the starting temperature. At the end of the second and third elapsed seconds from the start time, the rates of decrease are calculated, measuring over 2 and 3 seconds respectively, and in both cases the calculated rates are equal or more than 9° C./sec and 8° C./sec respectively. In all cases the calculated rate of decrease was equal or greater than the predetermined stored rates of decrease after the second and third second, so there is no “interference” with the normal thermostatic temperature control, and the current may be further boosted, while still bringing the final temperature of the 4 gramme emulator up to a maximum that falls below the threshold for heat damage. Five second form the start, FIG. 3 shows that the maximum temperature of the hair occurs immediately before the jaws are opened to release the hair, when the jaws are closed the second activation output from the activation sensor 22 signals the power control circuit 20 to stop monitoring the rate of change of temperature while, of course, still seeking to maintain the steady state temperature according to the setting of the input device 25.

FIGS. 4a and 4b show the same features as FIG. 3, but for a 2 gramme emulator, representing half the amount of hair which is normally styled, compared to FIG. 3, and thus an abnormally low thermal load. After the first second from closure of the jaws, the power control circuit 20 determines that the rate of decrease is less than 10° C./sec. Accordingly, there is a risk of overheating if the power input to the heating elements 13, 14 remains at the steady state level. The power control circuit 20 thus detects an abnormally low thermal load and responds, by operating the heating elements 13, 14 in a reduced mode at a power level lower than the steady state power level, specifically at a zero power level i.e. power supply to the elements is cut by the power control circuit 20. At the end of the second and third elapsed seconds from the start time, the rates of decrease are calculated, and in both cases the calculated rates are less than 9° C./sec and 8° C./sec respectively, so the power control circuit 20 maintains the heating elements 13, 14 at the power level lower than the steady state power level.

Likewise, if the power level is unchanged after the first second the power control circuit 20 is operative to reduce the power level applied to the heating element to zero following the second and third seconds from the activation signal, when either the sensed temperature exceeds the second calculated temperature (200° C. 18° C.=182° C.) and the measured rate of decrease is less than the second 9° C./sec reference rate of decrease after two seconds, or when the sensed temperature exceeds the third calculated temperature (200° C. 24° C.=176° C.) and the measured rate of decrease is less than the third 8° C./sec reference rate.

FIGS. 4a and 4b illustrate two options for control of the power supply to the elements following the operation at the power level lower than the steady state power level, and the jaws being opened (as occurs at Time=15 s in the two graphs). FIG. 4a shows the power control circuit 20 operating in the thermostatic power control mode and increasing the power according to the temperature offset or the difference between the instantaneous heating element temperature (approx. 164° C.) and the setpoint temperature (200° C.) according to the thermostatic no-load schema. However, as shown in FIG. 4a , it takes 3 or 4 seconds for the setpoint temperature to be attained, after use and the opening of the jaws. FIG. 4b illustrates a boosted control mode that boosts the element temperature more quickly than in the thermostatic mode. This boost mode may be selected by the power control circuit 20 following the operation at the power level lower than the steady state power level in order to recover more quickly. This predefined boost schema is followed after the second activation output indicates that the jaws are closed when operating at the power level lower than the steady state power level. According to the boost mode schema, for for all temperature offsets the power control circuit 20 operates to provide power to the elements at a higher level than for the thermostatic mode. The element power peaks at a higher value than with the thermostatic mode and, for instance, the power peak may correspond to 3 amperes, compared to 2 amperes in the conventional thermostatic mode. As shown, the result is that the element temperature is more quickly restored to the setpoint level—within about two seconds according to FIG. 4 b.

As illustrated in Table 1, the power control circuit 20 operates in the thermostatic mode for normal loads, during which operation the time rate of decrease of the element temperature (Tr) is monitored, and provided Tr is within the range of rates corresponding to the time from the first activation output between the (lower) predetermined rate of decrease and the (upper) reference rate of decrease, then no changes are made to the conventional thermostatic control schema. When Tr is below and above these ranges within the respective elapsed times then the power control circuit 20 operates in the reduced mode and boosted mode respectively. As described above, in the reduced mode the heating element is operated at a power level lower than the steady state power level for the current power setting. In the boost mode, the heating element is operated at a power level higher than the power level for the thermostatic mode e.g. in the boost mode the power directed to the elements may be 20 to 60% higher than that in the thermostatic mode, for the same measured difference between instantaneous element temperature and the setpoint temperature.

TABLE 1 Control mode selection according to rate of temperature decrease Measured rate of decrease of temperature Tr (° C./s) Control mode over 1 s over 2 s over 3 s Reduced Tr < 10 Tr < 9 Tr < 8 response Thermostatic 10 <= Tr <= 16 9 <= Tr <= 15 8 <= Tr <= 14 Boosted Tr > 16 Tr > 15 Tr > 14 response

The power control circuit 20 may also respond to an abnormally high thermal load, which tends to reduce the temperature of the elements at a faster rate than normal, operating in the boosted response mode to boost power in order that styling performance is not compromised. FIG. 5 illustrates such an abnormally high heat load on the appliance in the form of a 6 gramme hair emulator. For instance, memory in the power control circuit 20 may store the reference rates of decrease that are higher than the first predetermined rates of decrease. When the measured rate of decrease lies between the reference rate of decrease and the first predetermined rate of decrease (e.g. between 10 and 16° C./s after 1s), then the power control circuit 20 controls the heating elements 13, 14 in the thermostatic control mode, for instance power is increased in proportion to the difference between the measured element temperature and the setpoint temperature corresponding to the steady state power level. However, when the measured rate of decrease is greater than the reference rate of decrease (e.g. grater than 16° C./s after 1s), the control circuit operates the heating element in the boosted mode at successively higher boost power levels (above those defined by the conventional thermostatic control schema) until there is detected a measured rate of decrease of heating element temperature less than the reference rate, or a heating element temperature at a setpoint temperature corresponding to the steady state power output. Following this increase to higher boost power levels and the subsequent detection of a measured rate of decrease of heating element temperature less than the reference rate, the power control circuit 20 gradually decreases the power level applied to the heating element until the applied power level again equals the steady state power level or temperature, or until detection of an abnormally low thermal load recurs. When the steady state power level or temperature is reached, the power control circuit 20 reverts to the normal thermostatic control mode.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof. 

1. A hair styling appliance comprising: at least one heating element having a heating surface for contact with hair; a pair of jaws, wherein the heating element is disposed on a first of the jaws, and the jaws are connected for movement between an open position for inserting hair between the jaws and removing hair from between the jaws, and a closed position in which a second of the jaws presses the hair located between the jaws against the heating surface; a temperature sensor that senses temperature of the heating element proximate the heating surface; an activation sensor that senses when the jaws are in the closed position and generates an first activation output when the jaws are in the closed position; and a power control circuit that responds to the activation sensor and the temperature sensor, and operates the heating element at a steady state power level to maintain the temperature in a steady state, wherein the power control circuit includes means for determining a time rate of decrease of the temperature of the heating element, and means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load on the heating element, and the power control circuit, in response to the first activation output, starts detection of an abnormal thermal load, and, in response to detection of an abnormal thermal load, operates the heating element at a power level lower than the steady state power level.
 2. The hair styling appliance of claim 1, wherein the power level lower than the steady state power level is a zero power level.
 3. The hair styling appliance of claim 1, wherein the means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load comprises means for comparing the time rate of decrease of the temperature that is measured to a predetermined rate of decrease representing an abnormal thermal load, and the monitoring means detects an abnormal thermal load when the time rate of decrease that is measured is lower than the predetermined rate of decrease.
 4. The hair styling appliance of claim 1, wherein the means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load comprises means for comparing the temperature that is sensed to a first calculated temperature, a second calculated temperature that is lower than the first calculated temperature, and a third calculated temperature that is lower than the second calculated temperature, and means for comparing the rate of decrease of temperature that is measured to a first predetermined rate of decrease, a second predetermined rate of decrease that is less than the first rate of decrease and a third predetermined rate of decrease that is less than the second rate of decrease, and the monitoring means detects an abnormal thermal load when the temperature that is sensed is greater than the first calculated temperature and the rate of decrease that is measured is less than the first predetermined rate of decrease, or when the temperature that is sensed is less than the second calculated temperature and the rate of decrease that is measured is less than the second predetermined rate of decrease or when the temperature that is sensed is less than the third calculated temperature and the rate of decrease that is measured is less than the third predetermined rate.
 5. The hair styling appliance of claim 4, wherein the power control circuit reduces the power level applied to the heating element by the same amount When the temperature that is sensed is greater than the first calculated temperature and the rate of decrease that is measured is less than the first predetermined rate of decrease, When the temperature that is sensed exceeds the second calculated temperature and the rate of decrease that is measured is less than the second predetermined rate of decrease, or when the temperature that is sensed exceeds the third calculated temperature and the rate of decrease that is measured is less than the third predetermined rate.
 6. The hair styling appliance of claim 5, wherein the means for measuring the temperature rate of decrease comprises timing means for establishing a time interval between successive measurements of rates of decrease; and the timing means establishes first, second and third time intervals, measured from the time of a first activation output, and compares the rate rates of decreased that are measured and occur at ending of the first, second, and third time intervals to the first, second, and third predetermined rates of decrease, respectively.
 7. The hair styling appliance of claim 5, wherein the power control circuit stores a reference temperature corresponding to the steady state power, and the first, second, and third calculated temperatures are calculated by subtracting predefined first, second, and third reference values from the reference temperature to define the first, second, and third predetermined rates of decrease.
 8. The hair styling appliance of claim 1, wherein the control circuit operates the heating element in a thermostatic mode at first power levels, that vary as a function of a temperature offset to maintain a setpoint temperature, and the temperature offset comprises difference between instantaneous temperature of the heating element and the setpoint temperature, and the control circuit comprises means for comparing the rate of decrease to a reference rate of decrease, and, when the rate of decrease that is measured is greater than the reference rate of decrease, the control circuit operates the heating element in a boost mode at power levels higher that the first power levels, at all temperature offsets, until a rate of decrease of heating element temperature that is measured is less than the reference rate, or a heating element temperature is at a setpoint temperature corresponding to the steady state power level.
 9. The hair styling appliance of claim 8, wherein the activation sensor generates a second activation output when the jaws are in the open position and the power control circuit, in the presence of the second activation output, and following operation of the heating element at the power level lower than the steady state power level, operates the heating element in the boost mode until the power level again equals the steady state power level or until detection of an abnormal thermal load recurs.
 10. A method of controlling power in a hair styling appliances having a heating element and a temperature sensor that senses temperature of the heating element proximate a heating element surface, a pair of jaws, wherein the heating element is disposed on a first of the jaws, the jaws are connected for movement between an open position for inserting hair between and removing hair from between the jaws, and a closed position in which a second of the jaws presses the hair against the heating element, and an activation sensor that senses when the jaws are in the closed position and generates an first activation output when the jaws are in the closed position, the power control method comprising: operating the heating element at a steady state power level; when the first activation output is generated, periodically measuring the temperature of the heating element, computing a time rate of decrease of the temperature, and comparing the time rate of decrease that is computed to a predetermined rate, and when the time rate of decrease of the temperature that is measured is less than the predetermined rate, reducing the power level to the heating element to protect against hair damage by overheating. 